The invention relates to filters and in particular, to filters for the separation of impurities from waste gases from the exhaust gases of an internal combustion engine. The filter has a filter and high temperature resistant filter plates, in particular made from sintered metal powder, metal filings, metal fibres or a mixture thereof, which are disposed above and behind one another and between them form a plurality of flow ducts, whereby the flow ducts for forming inlet and outlet ducts are open on one side are closed on the other side, and whereby the walls of the filter plate lying between the inlet and outlet ducts represent filter surfaces.
A filter of this type is described for example in German Offenlegungsschrift 38 18 281. Filters are known in which the soot filter ducts are constructed very short, whereby the filter surface to a large-extent corresponds to the filter body flow-past surface. In this way the quantity of heat is produced with a burn-up of the soot adhering to the filter surface, can be reduced per filter body flow-past surface, as a result the filter system is prevented from being destroyed.
However in such a filter one disadvantage is that it has a very large diameter and for flow technology reasons also has a very large constructional volume because of the flow-past surface.
In principle it would also be possible to connect several smaller filter faces one behind the other and to allow the stream of waste gas to flow through them individually.
However a disadvantage of this is that the constructional expenditure required for this is very cost intensive.
In general one is faced with the problem that the soot filter plants and soot filter systems used at present have the disadvantage that the filter materials used have a lower thermal conductivity than massive materials because of their porosity. This means that with the partial burn-up of the soot in a duct the quantity of heat produced as a result can not be dissipated quickly enough. In this way there is the danger that materials fuse and thus the filter action is impaired. If the soot burns up in several ducts at the same time, such high thermal energy densities can occur in the filter, in particular in the rear region, that the entire filter melts through.
In particular when the filter plates have a poor thermal conductivity, the heat produced during the burn-up of the soot, during which combustion temperatures of over 200.degree. C. can be produced, is transferred to a large extent by the waste gas stream to the outlet of the filter and there results in a feared thermal energy densification which--as mentioned--causes the filter to burn through.
This meant that in practice a corresponding filter wall thickness had to be provided in order to obtain adequate stability and in order to increase the specific heat capacity. On the other hand the filter wall thickness should be reduced as much as possible in order to reduce the gas counter-pressure and therefore the costs.
The object of the invention is therefore to remove the disadvantages mentioned above, in particular to create a filter of the type mentioned at the beginning, in which the risk of a burning through is avoided as far as possible even with an optimal filter action or filter design.
This object is achieved in accordance with the invention in that heat conducting sheets are inserted into the inlet ducts.